This invention relates to the comminution of minerals and, more particularly, but not exclusively, is concerned with the comminution of the harder varieties of natural calcium carbonate such as calcite marble, vein calcite, dolomite and limestone, in order to provide fine particulate calcium carbonate suitable for use as a pigment or filler material.
Conventionally the harder types of calcium carbonate are quarried by blasting and the large lumps of rock thus liberated are broken down in heavy duty crushers. The final product of the crushing plant is generally chippings having a maximum particle size of about 20 mm. These chippings are then further reduced in size by milling in a ball mill charged with a suitable grinding medium which may conveniently be flint pebbles having diameters of about 50-100 mm although other grinding media may equally well be used. The ball milling step may be performed wet or dry but when the product is required to be comminuted further it is preferred to grind the chippings in the form of an aqueous slurry.
The ball mills conventionally used have the disadvantages that their capital cost is high and that the amount of energy which can be brought to bear on the feed material is governed by the weight of the grinding medium and the maximum distance through which they can fall under gravity which is generally a little less than the internal diameter of the mill. For this reason the grinding efficiency of a ball mill, measured in terms of the weight of feed per hour which can be reduced to a given particle size, tends to be rather low. In addition, the cost of maintaining a conventional ball mill may be high because it is supported at each end in heavy roller or ball bearings which are subject to wear and must be renovated at frequent intervals. Also, while grinding the feed, the grinding balls or pebbles themselves become abraded or fractured to some extent and the product may therefore become contaminated with the material of which the pebbles or balls are constituted.